Windows 10 Mobile on older hardware: how is it?

You can force Windows 10 to install on older Windows Phone 8.1 smartphones but should you?
When Windows 10 Mobile launched, Microsoft controversially decided not to include support for older Windows Phone devices because of how CPU intensive Windows 10 Mobile was. Throughout the preview program, users using older Windows Phones had constant complaints with slowness, laggyness and general bugs that were not helped by the aging Snapdragon S4 processors.